A Nonet is a 9-line poem that has 9 syllables in the first line, 8 syllables in the second line, 7 syllables in the third line, and continues to count down to one syllable in the final (ninth) line.
For me, visual aesthetics are important for a Nonet. I typically chose a centered one. I use a syllable counter. A good one is: howmanysyllables.com.